Various kinds of illuminating devices comprising phosphors have conventionally been used as illuminating devices emitting white light. In recent years, however, new light sources comprising inorganic EL (electro luminescence), organic EL (OLED (organic light emitting diode)), and semiconductor light-emitting elements such as light-emitting diodes have been developed, and illuminating devices employing these light sources are also being developed.
For example, patent document 1 discloses an illuminating device which comprises three kinds of light-emitting diodes, as light-emitting devices, that emit light of blue, green, and red colors respectively and in which white light is obtained by mixing the light emitted from the three kinds of light-emitting diodes. The illuminating device disclosed in patent document 1 further comprises a current regulation circuit with which values of operating current to be supplied to the three kinds of light-emitting diodes are changed according to forward voltages applied to the three kinds of light-emitting diodes. This illuminating device has been configured so that a white balance can be regulated according to the characteristics of each light-emitting diode.
However, light-emitting diodes have a relatively narrow emission spectrum width and, hence, illuminating devices employing light-emitting diodes as light-emitting devices disadvantageously have reduced color rendering properties, which are important in general illumination.
Patent document 2 discloses an illuminating device employing light-emitting devices in which light from a light-emitting diode is changed in wavelength with a phosphor. Specifically, patent document 2 shows an example of an illuminating device which comprises (a) a blue-light-emitting device comprising a blue-light-emitting diode, (b) a green-light-emitting device based on a combination of a blue-light-emitting diode and a green phosphor which is excited by blue light from the blue diode to emit green light, and (c) a red-light-emitting device based on a combination of a blue-light-emitting diode and a red phosphor which is excited by blue light from the blue-light-emitting diode to emit red light. By thus converting the emission from a light-emitting diode in wavelength with a phosphor, color rendering properties are improved. Patent document 2 further describes changing emission color by regulating the output of the light-emitting devices for the respective colors.
LEDs are expected to be used also as light sources for tone-variable illumination, which has been difficult to realize with conventional light sources. Disclosed as one example thereof is an illuminating device in which a red LED, a green LED, and a blue LED have been configured to form one package to thereby output white light (see, for example, patent document 3). This technique includes a contrivance in which an operating current to be supplied to the three kinds of LEDs is regulated according to a forward voltage applied to each LED to thereby keep the emission efficiency of each LED constant and stabilize the luminance of white light and to thereby emit light having various color tones.
As an illumination technique utilizing LEDs, a technique has been disclosed in which semiconductor light-emitting devices emitting red, blue, and green light using blue LEDs and phosphors for emitting red and green light are used in combination and the output of the LEDs is controlled to thereby trace a blackbody radiation locus and emit white light akin to natural light (see, for example, patent documents 3 to 5, non-patent document 1, etc.). In patent documents 6 to 10 also, illuminating devices employing various LEDs are proposed.    Patent Document 1: JP-A-2006-4839    Patent Document 2: JP-A-2007-122950    Patent Document 3: JP-A-2007-59260    Patent Document 4: JP-A-2007-265818    Patent Document 5: JP-A-2007-299590    Patent Document 6: JP-A-2007-27310    Patent Document 7: JP-A-2005-57272    Patent Document 8: JP-A-2006-310613    Patent Document 9: JP-A-2007-80880    Patent Document 10: JP-A-2007-266314    Non-Patent Document 1: http://techon.nikkeibp.co.jp/article/NEWS/20070704/135373/(articles on Tech-on in Nikkei BPnet)